The app fetches the page itself and pulls out the readable text. Pages behind a login or a paywall may refuse — for those, open the page, copy the text, and use Paste.
Supported: PDF (with a text layer), DOCX, PPTX, TXT, MD, HTML — up to 25 MB. Scanned PDFs can't be read (this version has no OCR), and spreadsheets aren't supported.